Deep Copy Nested Collections

Basics

Easy

list

dictionary

collections

Deep Copy Nested Collections

A colleague "fixed" a data-mixup bug by duplicating a nested schedule structure with := — and a week later, edits made to the copy were mysteriously appearing in the original. Nothing mysterious about it: in AL, List and Dictionary are reference types. Assigning one variable to another copies the reference, not the collection — both variables read and write the same data. Even GetRange, which does build a new list, performs only a shallow copy: when the elements are themselves lists, the new outer list still points at the very same inner lists.

Your job is a small utility codeunit that produces genuinely independent copies of two nested collection shapes.

Requirements

Create a codeunit named "Collection Deep Copy" with two public procedures:

procedure CopyMatrix(Source: List of [List of [Integer]]): List of [List of [Integer]]
procedure CopyGroups(Source: Dictionary of [Code[20], List of [Text]]): Dictionary of [Code[20], List of [Text]]

Rules:

CopyMatrix returns a new outer list containing a new inner list for every inner list of Source, with the same integers in the same order.
CopyGroups returns a new dictionary with the same keys, each mapped to a new list holding the same texts in the same order.
The copies must be fully independent of the source: after copying, adding values to an inner list (or to a group's list) on either side must never show up on the other side.
Empty inner lists count too: an empty inner list in the source comes out as an empty — and equally independent — inner list in the copy.
Copying must not modify Source.

Pick your object ID in the 50100–50199 range and reference other objects by name, never by ID.

What the tests check

The grading tests copy matrices and dictionaries filled with generated values, then compare counts, keys, and every element in order — including a source that contains an empty inner list, which must come out empty. After each copy they mutate an inner list on one side (the copy in some tests, the source in others, previously-empty lists included) and assert the other side is unchanged — a solution that shares any inner list between source and copy fails those tests.

Learn More

List data type — why a List is a reference type, and what shallow versus deep copy means for nested lists.
Dictionary data type — reference semantics plus the key/value methods you will need.
List.GetRange method — the documented shallow-copy method and its limits.
Dictionary.Keys method — iterate a dictionary's keys to rebuild it.

Hint 1

List and Dictionary are reference types: := gives you a second variable pointing at the same collection, and GetRange copies only the outer list — the inner lists inside it are still the very same objects.

Hint 2

A real deep copy visits every element: loop over the outer collection, and for each inner list build a brand-new list and fill it with that list's values one by one.

Hint 3

A small local helper that takes an inner list and returns a freshly built copy keeps each public procedure to a single foreach — and because a new list is created on every call, empty inner lists come out empty and independent for free.

ALBusiness Central 28.4

Press Compile to check your code compiles — Submit runs the tests.

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Open this problem on a computer to write and run code. Reading the description, tests and discussion works fine here.